1. Field of the Invention
The present invention relates to a can-type secondary battery, and more particularly to a can-type secondary battery adapted to induce a short circuit between metal within the secondary battery when the secondary battery deforms due to an external impact
2. Description of the Prior Art
As portable wireless appliances including video cameras, portable telephones, and portable computers tend to have reduced weight while incorporating more functions, much research has been conducted on secondary batteries which are used as the driving source of the appliances. For example, secondary batteries include nickel cadmium batteries, nickel hydrogen batteries, nickel zinc batteries, and lithium secondary batteries. Lithium secondary batteries are widely used in the cutting-edge electronic appliance fields because they can be recharged, they can be manufactured in a compact size with large capacity, and they have high operating voltage and high energy density per unit weight.
FIG. 1 is an exploded perspective view showing a conventional can-type secondary battery.
The can-type secondary battery is formed by placing an electrode assembly 112 including first and second electrode plates 115, 113 and a separator 114 into a can 110 together with an electrolyte and sealing the top opening 110a of the can 110 with a cap assembly 120. The first and second electrode plates 115, 113 may be formed as negative and positive electrode plates, respectively.
The cap assembly 120 includes a cap plate 140, an insulation plate 150, a terminal plate 160, and an electrode terminal 130. After being coupled to a separate insulation case 170, the cap assembly 120 is coupled to the top opening 110a of the can 110 to seal it.
The cap plate 140 is a metal plate having a size and a shape corresponding to the top opening 110a of the can 110. The cap plate 140 has a first terminal through-hole 141 formed at the center thereof with a predetermined size, into which the electrode terminal 130 is insertable. When the electrode terminal 130 is inserted into the first terminal through-hole 141, a gasket tube 146 is coupled to the outer surface of the electrode terminal 130 and is inserted together for insulation between the electrode terminal 130 and the cap plate 140. The cap plate 140 has an electrolyte injection hole 142 formed on the other side thereof with a predetermined size. After the cap assembly 120 is assembled to the top opening 110a of the can 110, an electrolyte is injected through the electrolyte injection hole 142, which is then sealed by a plug 143.
The electrode terminal 130 is connected to the first electrode tab 117 of the first electrode plate 115 or to the second electrode tab 116 of the second electrode plate 113 acts as a negative terminal or positive terminal.
The insulation plate 150 is made up of an insulation material like the gasket and is coupled to the bottom surface of the cap plate 140. The insulation plate 150 has a second terminal through-hole 151 formed in a position corresponding to the first terminal through-hole 141 of the cap plate 140 so that the electrode terminal 130 can be inserted therein. The insulation plate 150 has a seating groove 152 formed on the bottom surface thereof with a size corresponding to that of the terminal plate 160 so that the terminal plate 160 may be seated thereon.
The terminal plate 160 is made up of Ni metal or an alloy thereof and is coupled to the bottom surface of the insulation plate 150. The terminal plate 160 has a third terminal through-hole 161 formed in a position corresponding to the first terminal through-hole 141 of the cap plate 140 so that the electrode terminal 130 may be inserted therein. The electrode terminal 130 is coupled to the terminal plate 160 via the first terminal through-hole 141 of the cap plate 140 while being insulated by the gasket tube 146. As such, the terminal plate 160 is electrically connected to the electrode terminal 130 while being electrically insulated from the cap plate 140.
In order to couple the electrode terminal 130 to the cap plate 140, the insulation plate 150, and the terminal plate 160, the electrode terminal 130 is rotated while applying a constant force and is inserted into the first terminal through-hole 141. After passing through the first terminal through-hole 141, the electrode terminal 130 successively passes through the second and third terminal through-holes 151, 161, which are formed on the insulation plate 150 coupled to the bottom surface of the cap plate 140 and on the terminal plate 160, respectively. The inner diameter of the second terminal through-hole 151 formed on the insulation plate 150 is equal to or slightly larger than the outer diameter of the inserted electrode terminal 130 so that the electrode terminal 130 may be press-fitted into the second terminal through-hole 151 while the outer surface of the electrode terminal 130 is fastened thereto.
When an internal or external short circuit occurs in the electrode assembly of the lithium ion secondary battery or when the battery is subjected to overcharging/over-discharging, the voltage of the battery may rise abruptly and the battery may fracture. In order to avoid a short circuit within the secondary battery, insulation tape may be attached to parts vulnerable to a short circuit, including the welded portions between the electrode tab and the ends of the first and second electrode plates of the electrode assembly. In addition, the secondary battery is electrically connected to safety devices including a positive temperature coefficient (PTC) thermistor, a thermal fuse, and a protective circuit, in order to interrupt current when the voltage or temperature of the battery rises abruptly and to prevent the battery from fracturing.
When the secondary battery deforms due to an impact or pressure, however, neither the protective circuit nor the protective device may be able to avoid a short circuit between the electrode plates. According to a longitudinal compression evaluation method, which is one of the methods for evaluating the safety of the can-type secondary battery, the short circuit between the electrode plates within the can-type secondary battery is a problem. In a longitudinal compression test, which is one of the items for evaluating the safety of the can-type secondary battery, a compression jig is used to compress both lateral surfaces of the can-type secondary battery in a direction perpendicular to the longitudinal direction of the can-type secondary battery. During the compression, the compression surfaces of the compression jig remain parallel to both lateral surfaces of the can-type secondary battery and the compression force is 13 kN. As the can-type secondary battery is compressed according to the longitudinal compression evaluation method, the first and second electrode plates are short-circuited and current flows abruptly from the second electrode plate to the first electrode plate. As a result, excessive heat is generated by the first and second electrode plates' own resistance. The excessive heating may cause the secondary battery to explode.